Use of uncalcined/partially calcined ingredients in the manufacture of sintered pellets useful for gas and oil well proppants

ABSTRACT

A composite, sintered, spherical pellet and a method for its manufacture are described. The pellet comprises clay and a member of the group of bauxite, alumina, or mixtures thereof; the pellet being prepared from at least one uncalcined or partially calcined ingredient. The pellet may have an alumina-to-silica ratio from about 9:1 to about 1:1. The pellet has a specific gravity of less than 3.40. Use of such pellets in propping hydraulically fractured subterranean formations is also described.

This is a continuation-in-part of Ser. No. 405,055, filed Aug. 4, 1982,now U.S. Pat. No. 4,427,068, which is a continuation-in-part of Ser. No.347,210, filed Feb. 9, 1982, abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to oil and gas well proppants and, moreparticularly, to sintered proppants made from ingredients at least someof which are uncalcined or partially calcined, a method of making suchproppants, and to a method of maintaining a fracture in a subterraneanformation in a propped condition by utilizing such proppants.

2. History of the Prior Art

Oil and natural gas are produced from wells having porous and permeablesubterranean formations. The porosity of the formation permits theformation to store oil and gas, and the permeability of the formationpermits the oil or gas fluid to move through the formation. Permeabilityof the formation is essential to permit oil and gas to flow to alocation where it can be pumped from the well. Sometimes thepermeability of the formation holding the gas or oil is insufficient foreconomic recovery of oil and gas. In other cases, during operation ofthe well, the permeability of the formation drops to the extent thatfurther recovery becomes uneconomical. In such cases, it is necessary tofracture the formation and prop the fracture in an open condition bymeans of a proppant material or propping agent. Such fracturing isusually accomplished by hydraulic pressure, and the proppant material orpropping agent is a particulate material, such as sand, glass beads orceramic pellets, which are carried into the fracture by means of afluid.

Spherical pellets of uniform size are believed to be the most effectiveproppants due to maximized permeability. For this reason, assuming otherproperties to be equal, spherical or essentially spherical proppants,such as rounded sand grains, metallic shot, glass beads and tabularalumina, are preferred.

In practice, in deep wells, where high pressures are encountered, e.g.,above about 700 kg/cm² (10,000 psi), the immediately foregoingspecifically mentioned proppants are either entirely ineffective or donot exhibit desired permeability. Examples of prior art proppants andtheir use are found in U.S. Pat. Nos. 2,950,247, McGuire, et al;3,026,938, Huitt, et al; 3,126,056, Harrell; 3,497,008, Graham, et al;3,976,138, Colpoys, et al; and 4,068,718, Cooke, et al. One of thebetter proppants useful at high pressures, disclosed in U.S. Pat. No.3,976,138, is predominantly alumina. However, even such alumina, asdisclosed in U.S. Pat. No. 3,976,138, has reduced permeability atpressures in excess of 350 Kg/cm² (5,000 psi).

As disclosed in U.S. Pat. No. 4,068,718, sintered bauxite made fromcalcined bauxite unexpectedly has a permeability which is superior tothe previously mentioned proppant materials at pressures as high as 700kg/cm² (10,000 psi) or higher. Pellets having a high apparent specificgravity, i.e. greater than 3.4, are disclosed in U.S. Pat. No. 4,068,718to be most suitable as proppant materials.

The prior art sintered bauxite particles made from calcined bauxite maybe produced in spherical shape as described in R. J. Seider's commonlyassigned, copending U.S. Patent Application Ser. No. 252,491, filed Apr.8, 1981 abandoned, as a continuation of U.S. Patent Application Ser. No.969,122, filed Dec. 13, 1978 abandoned. Such prior art sintered bauxiteproppants prepared from fully calcined bauxite, although extremelyuseful under high pressure conditions, over about 700 kg/cm² (10,000psi), are costly. The cost of the prior art high strength, sintered,calcined bauxite proppant for wells of intermediate pressures, betweenabout 350 and 700 kg/cm² (5,000 and about 10,000 psi), may not beeconomically justified.

The manufacture of sintered spherical pellets from calcined clay andcalcined bauxite, calcined alumina or mixtures thereof, is described inJ. F. Fitzgibbon's commonly assigned, copending U.S. Patent ApplicationSer. Nos. 347,210 filed Feb. 9, 1982 abandoned, and 405,055 filed Aug.4, 1982 now U.S. Pat. No. 4,427,068. These pellets are available atsomewhat lower cost and are aptly suited to use under pressures of up toabout 700 Kg/cm² (10,000 psi). These pellets have lower specificgravities and bulk densities than those made according to U.S. Pat. No.4,068,718.

Calcining adds considerably to the cost of the raw materials oringredients used in the manufacture of pellets useful as proppant. Forexample, the cost of dried diaspore clay is about 35 dollars per ton, ifair dried. The cost of the same material is about 70 dollars per ton, iffully calcined.

While the prior art ceramic pellets made from fully calcined ingredientsare aply suited for many proppant applications there remains a need toprovide strong ceramic pellets that are of even lower cost.

The present invention provides strong pellets aptly suited to use asproppants under pressures of up to about 700 kg/cm² (10,000 psi), whichare more economical than previously available synthetic ceramicproppants and have low specific gravities and bulk densities, whichbenefit the user, in that fewer pounds of proppant per cubic foot offracture are required. Handling, e.g., pumping of slurries of lowdensity material, is easier than handling of high density materials.

BRIEF DESCRIPTION OF THE INVENTION

In accord with the present invention, composite, spherical pellets orparticles containing one or more uncalcined or partially calcinedingredients as a component and having an alumina-to-silica dry weightbasis ratio of from about 9:1 to about 1:1 and apparent specificgravities less than 3.30, are produced. Diaspore clay, burley clay andflint clay have been found to be useful in the manufacture of suchpellets although it is believed that other clays may be employed.Surinam and Comalco bauxites have been found to be useful although it isbelieved that other bauxites may be employed. Such spherical particleshaving an alumina to silica dry weight basis ratio of from about 9:1 to1:1 and an apparent specific gravity of at least about 2.6 are useful asoil and gas well proppants.

The present uncalcined or partially calcined materials which areparticularly adapted to use in combination with known, prior artproppant materials include fines from the dust collection systems ofclay calcining kilns, and uncalcined or partially calcined clays anduncalcined or partially calcined bauxites. These uncalcined or partiallycalcined materials are blended with fully calcined clays and fullycalcined bauxites or alumina to produce composite sinterable, sphericalpellets which are subsequently furnaced to produce sintered, sphericalpellets eminently useful as proppants. The composites of the presentinvention may be made from a significant portion and may even be madefrom a major portion of an uncalcined or partially calcined ingredientor ingredients. Composites of the present invention may be made from avery small amount of uncalcined or partially calcined ingredients butpreferably are made from at least about five (5) percent by weight ofuncalcined or partially calcined ingredients.

The partially calcined and uncalcined clay and bauxite materials of thepresent invention are compatible with, and may be formed into a matrixwith, a wide variety of proppant materials, and, in this manner, a widevariety of composite proppants may be produced, which may be customizedto particular conditions or formations. Thus, the properties of thefinal sintered composite pellets, such as strength, permeability,specific gravity, bulk density and acid resistance, may be controlledthrough variations in the initial component mixture.

Combinations of dust collector fines, diaspore clay and bauxite areparticularly useful. Such mixtures may suitably contain up to 70 percentby weight uncalcined clay. Mixtures containing up to 50 percent byweight uncalcined clay have a broad range of use, and mixturescontaining up to 40 percent by weight uncalcined clay have aparticularly broad range of use.

The present invention also provides a process for propping fractures inoil and gas wells at depths of 6,000 to 14,000 feet utilizing thepresent sintered pellets by mixing the pellets with a fluid, such as oilor water, and introducing the mixture into a fracture in a subterraneanformation. The compaction pressure upon the fracture generally is atleast 280 kg/cm² (4,000 psi) and usually is in the range of from about350 to about 700 kg/cm² (5,000 to about 10,000 psi). The present pelletshave an average particle size between 0.1 and 2.5 millimeters. It hasbeen found that the present composite pellets containing 50 percent ormore parts by weight uncalcined clay, at pressures up to about 700kg/cm² (10,000 psi), have desirable permeability characteristics, i.e.,they exhibit a permeability to brine at about 93.3° C. (200° F.) whichdecreases not more than about three-fourths when the pressure applied tothem is increased from 140 to 700 kg/cm² (2,000 to 10,000 psi).

The present proppant materials are produced by forming a mixture ofdried but uncalcined or only partially calcined clays and bauxites anddust collector fines with fully calcined materials. The startingingredients have an average particle size of less than about 15 micronsand, preferably, less than about 10 microns and, most preferably, lessthan about 5 microns.

In a preferred method, the mixture is produced on an intensive mixerhaving a rotatable table provided with a rotatable impacting impeller,such as described in U.S. Pat. No. 3,690,622, to Brunner. Sufficientwater is added to cause essentially spherical ceramic pellets to form,and, after such pellets have formed, from about 5 to about 15 percent ofadditional ceramic powder by weight of pellets is added, and the mixeris further operated to cause accretion of the added material to thepellets being formed.

The resulting pellets are then dried to a moisture content of less thanten (10) weight percent, usually at between about 100 and about 300degrees centigrade, and thereafter furnaced at sintering temperatureuntil an apparent specific gravity between about 2.60 and about 3.30 isobtained, depending on the composition of the sintering mixture.

DETAILED DESCRIPTION OF THE INVENTION

The sintered composite proppant pellets of the present invention haveapparent specific gravities less than 3.30. Preferably they spherical inshape.

The sphericity of the pellets may be determined using a visualcomparator. Krumbein and Sloss, Stratigraphy and Sedimentation, secondedition, 1955, W. H. Freeman & Co., San Francisco, CA, describe a chartfor use in visual determination of sphericity and roundness. Visualcomparison using this chart is a widely used method of evaluatingsphericity or roundness of particles. In using the visual comparisonmethod, a random sample of 10 to 20 particles of the material to betested is selected. The particles are viewed under a 10 to 20 powermicroscope or photomicrograph and their shapes compared to the Krumbeinand Sloss chart. The chart values for sphericity range from 0.3 to 0.9.The chart values for the individual particles are then averaged toobtain a sphericity value. The present particles have an averagesphericity of about 0.7 or greater when visually compared with theKrumbein and Sloss chart.

"Spherical" and related forms, as used herein, is intended to mean anaverage ratio of minimum diameter to maximum diameter of about 0.70 orgreater, or having an average sphericity value of about 0.7 or greatercompared to a Krumbein and Sloss chart.

"Bulk density", as used herein, is the weight per unit volume, includingin the volume considered, the void spaces between the particles.

"Apparent specific gravity" is a number without units, but numericallyequal to the weight in grams per cubic centimeter of volume, excludingvoid space or open porosity in determining the volume. The apparentspecific gravity values given herein were determined by liquid(ethylbenzene) displacement.

"Theoretical density" and "true specific gravity" exclude not only thevoid space between particles and open porosity of particles from thevolume, but also exclude closed porosity. These latter two measures arenot customarily used for characterizing proppants. Theoretical densityand true specific gravity require fine grinding to expose any closedporosity.

"Calcined" as used herein, refers to a process to which a material hasbeen subjected. Ore materials that have been fully subjected tocalcination or a calcining process exhibit very low loss on ignition(LOI) and moisture contents, e.g. about 1-2 percent by weight or less.Uncalcined ore materials such as bauxites and clays can contain fromabout 10 to about 40 percent by weight volatiles. "Partially calcined"materials such as diaspore clay calcination kiln dust collection systemfines typically exhibit total volatiles (LOI plus moisture content) of 5to 8 percent by weight. Volatiles can include moisture, organics andchemically held water (eg water of hydration).

"Dust collector fines" as used herein refers to material obtained fromthe dust collection system of a calcining kiln operating on a clay orbauxite ore. Such fines are usually partially calcined and exhibit ahigher silica (SiO₂) content than the ore being processed in the kiln.

Unless otherwise stated at the point of interest, all percentages,proportions and values with respect to composition are expressed interms of weight.

The sintered, spherical pellets of the present invention may bemanufactured by furnacing a mixture of calcined and partially calcinedor uncalcined ingredients. While various sintering aids, such asbentonite clay or iron oxide, boron, boron carbide, aluminum diboride,boron nitride, boron phosphide and other boron compounds, and fluxes,such as sodium carbonate, lithium carbonate, feldspar, manganese oxide,titania, iron oxide and sodium silicates, may be added in amounts up toabout ten (10) weight percent to aid sintering, such additions aregenerally unnecessary because the use of partially or totally uncalcinedingredients promotes sintering at lower temperatures than thosenecessary to obtain finished pellets having comparable properties whenusing only fully calcined ingredients. If desired, a binder, forexample, various resins or waxes known in the prior art, may be added tothe initial mixture to improve pelletizing and to increase the greenstrength of the unsintered pellets.

Pellets according to the present invention and for use with the presentinvention may be prepared from a mixture of any of the clays describedin Tables I and II with one or more bauxites or alumina or mixtures ofthese. The composition of specific bauxites employed in the manufactureof the pellets described herein is also given in Tables I and II. Allvalues in Tables I and II are expressed as percentages by weight. Wherean omission occurs, it indicates that sufficient testing was notconducted to obtain a value.

                                      TABLE I    __________________________________________________________________________    (Typical Analysis of Ingredients)    Surinam  Control                  Uncalcined Clay                            Uncalcined Clay                                      Uncalcined Clay                                               (Dust Collector Fines)    Bauxite  Batch                  (16 Mesh and Finer)                            (1982 Stockpile)                                      (Stockpile #1)                                               Samp. 1                                                    Samp. 2                                                         Samp.                                                              Samp.    __________________________________________________________________________                                                              4    H.sub.2 O        0.57 .611 0.88      0.88      0.81     --   --   --   1.51    LOI 1.23 1.26 11.12     11.12     12.75    6.20 6.27 6.07 6.51    Al.sub.2 O.sub.3        86.96             71.6 51.84     51.84     58.53    45.52                                                    45.44                                                         47.33                                                              58.41    SiO.sub.2        2.51 16.929                  37.87     37.87     29.53    41.00                                                    41.08                                                         39.76                                                              26.18    Fe.sub.2 O.sub.3        5.95 4.84 1.04      1.04      1.00     2.30 2.29 2.49 3.34    TiO.sub.2        3.35 3.06 2.65      2.65      3.09     4.08 4.06 4.28 4.05    Na.sub.2 O        --   1.32 .021      .021      .009     1.41 1.37 1.91 --    K.sub.2 O        --   .94  4.25      4.25      2.82      0.055                                                     0.017                                                          0.049                                                              --    CaO --   .08  0.14      0.14      0.15     1.42 1.42 1.30 --    __________________________________________________________________________

                  TABLE II    ______________________________________    (Typical Analysis of Ingredients - calcined)                    High    High  High                    Purity  Silica                                  Iron    Chemical        Dis-    Dis-  (Brown)    Com-   Surinam  apore   apore Disapore                                         Burley                                               Flint    pound  Bauxite  Clay    Clay  Clay   Clay  Clay    ______________________________________    Al.sub.2 O.sub.3           86.80    75.10   70.00 78.30  54.07 38.52    SiO.sub.2           3.42     18.60   24.40 15.09  41.33 56.64    Fe.sub.2 O.sub.3           4.74     0.80    0.80  2.63   1.26  0.65    TiO.sub.2           3.13     2.99    3.04  3.05   2.45  3.49    Other  1.00     1.51    --    0.72   0.74  0.67    (e.g.    MgO,    CaO)    Loss on           0.91     1.00    --    0.21   0.15  0.03    Ignition    Moisture    Apparent           3.6-3.7  2.9-3.1 2.9-3 2.9-3.1                                         2.7-2.8    2.5-2.6    Specific    Gravity    After    Calcining    g/cc    ______________________________________

Each of the clays and dust collector fines described in Tables I and IImay be obtained from Missouri Minerals Processing, High Hill, Mo. 63350,in raw or calcined form.

The Surinam bauxite described in Table I may be obtained from AluminumCompany of America, Pittsburgh, Pa. 15219. Surinam bauxite is sodesignated for that is the country in which it is mined. It is expectedthat other bauxites may be used without departing from the presentinvention.

Diaspore clays, as found in nature, are predominantly hydrated aluminumoxide (Al₂ O₃.H₂ O). Such clays occur in emery rock in association withcorundum. The main deposits of diaspore clays in the United States arein Missouri and Pennsylvania. Diaspore clays have a hardness between 6.5and 7.0 and a true specific gravity usually between 3.30 and 3.45gm/cm³. The crystal structure of diaspore clay is orthorhombic.Typically, diaspore clay, as found in nature, contains 25 to 30 percentby weight, and, in some cases, as high as 35 percent by weight, ofimpurities. Generally, the major impurities are: SiO₂, which typicallyranges from about 12 to about 25 percent by weight; TiO₂, whichtypically ranges from about 2.75 to 3.75 percent; Fe₂ O₃, typicallybetween 0.25 and 1.0 percent, and MgO and CaO, generally less than 1.0percent.

Dust collector fines sample 4 was generated while calcining diasporeclay. The materials which were being calcined during generation of dustcollector fines samples 1-3 are not known.

The uncalcined clay and bauxitic materials for use in the presentinvention are usually air dried at low temperature, e.g. 90°-150° C.(200°-300° F.) prior to use with other calcined ingredients. Air dryingremoves free moisture; that is, moisture that is not chemicallycombined. The calcined ingredients are initially calcined, by knownprior art methods, at temperatures and times sufficiently high,typically 1000°-1200° C., to remove any organic material and to removemost or all water of hydration. Water of hydration is chemicallycombined water. Calcined and partially calcined materials may be usedwithout further treatment unless they have been stored in a manner thatpermits pickup of free moisture, in which circumstance they should bedried in air or at low temperature prior to use. Free water or moistureis not chemically combined. Excess free moisture may cause agglomerationand caking during ball milling of the ingredients. Ball milling isnormally employed to reduce the particle size of the ceramic ingredientsthe desired small size.

The sintered pellets of the present invention are preferably made asfollows:

1. Starting ingredients of uncalcined or partially calcined clay andcalcined clay and uncalcined or partially calcined bauxite and calcinedbauxite or alumina, or mixtures thereof, are added in a predeterminedratio to a high intensity mixer. The ratio of ingredients is chosenbased on analysis of the ingredients and desired analysis and specificgravity of the pellets to be produced. Preferably, at least five (5)percent of the total ingredients on a dry weight basis is uncalcined orpartially calcined material. Each of the ceramic ingredients has anaverage particle size of less than about 15 microns and preferably lessthan about 10 microns and most preferably, less than about 5 microns.

Small particle size is required in order to obtain a finished sphericalsintered pellet having the desired density. An average particle size ofsmaller than 5 microns is desirable, and the average particle size ismost preferably below 3 microns and usually above 0.5 microns.

2. The powdered ceramic starting ingredients are stirred to form a dryhomogeneous particulate mixture having an average particle size of lessthan about 15 microns.

A preferred stirring or mixing device is that obtainable from EirichMachines, Inc., known as the Eirich Mixer. A mixer of this type isprovided with a horizontal or inclined circular table, which can be madeto rotate at a speed of from about 10 to about 60 revolutions per minute(rpm), and is provided with a rotatable impacting impeller, which can bemade to rotate at a tip speed of from about 5 to about 50 meters persecond. The direction of rotation of the table is opposite that of theimpeller, causing material added to the mixer to flow over itself incountercurrent manner. The central axis of the impacting impeller isgenerally located within the mixer at a position off center from thecentral axis of the rotatable table. The table may be in a horizontal orinclined position, wherein the incline, if any, is between 0 and 35degrees from the horizontal.

3. While the mixture is being stirred, there is added sufficient waterto cause formation of composite, spherical pellets from the ceramicpowder mixture.

In general, the total quantity of water which is sufficient to causeessentially spherical pellets to form is from about 17 to about 20percent by weight of the initial starting ceramic ingredients andusually between about 18 and about 20 percent by weight of the initialceramic powder. The total mixing time usually is from about 2 to about 6minutes.

After the clay mixture is added to the mixer, the table is rotated atfrom about 10 to about 60 rpm and, preferably, from about 20 to about 40rpm, and the impacting impeller is rotated to obtain a tip speed of fromabout 25 to about 50, preferably, from about 25 to about 35, meters persecond, and sufficient water is added to cause essentially sphericalpellets of the desired size to form. If desired, the impeller may beinitially rotated at a lower tip speed of from about 5 to about 20meters per second during addition of the first half of the sufficientwater and subsequently rotated at the higher tip speed of 25 to about 50meters per second during the addition of the balance of the water. Therate of water addition is not critical. The intense mixing actionquickly disperses the water throughout the particles.

4. The resulting pellets are dried at a temperature well below sinteringtemperature until less than 10 percent, preferably less than 3 percentand, most preferably, less than 1 percent free moisture remains in thepellets. Drying is preferably done in a rotary kiln with flowing gas ata temperature of between about 100° (212° F.) and about 300° C. (572°F.). The most preferred drying gas temperature is between about 175°(347° F.) and 275° C. (527° F.), and the drying time required is usuallybetween about 30 and about 60 minutes. The pellets themselves aregenerally at a lower temperature than that of the heated gas used to drythem.

5. The dried pellets are then furnaced at sintering temperature for aperiod sufficient to enable recovery of sintered, spherical pelletshaving an apparent specific gravity of between 2.70 and 3.30 and a bulkdensity of from about 1.35 to about 1.80 grams per cubic centimeter. Thespecific time and temperature to be employed is, of course, dependent onthe ingredients employed and the optimum time and temperature for agiven starting composition is determined empirically according to theresults of physical testing of the resulting pellets after furnacing.

The furnacing step is carried out to sinter the composite pellets;generally, temperatures of between about 1,300° C. (2,372° F.) and about1,500° C. (2,732° F.) for about 4 to about 20 minutes and, morepreferably, from about 1,375° (2,498° F.) to about 1,435° C. (2,606° F.)for about 4 to about 8 minutes, are useful, depending upon the sinteringaids and fluxes which may be included or naturally present in theingredients.

While the process just described hereinabove will yield pelletsaccording to the invention, it is preferred that from about 5 to about15 percent and, preferably, from about 8 to about 10 percent ofadditional starting ingredients by weight of pellets be added, after theaddition of water but prior to drying of the pellets. The added materialis usually but not necessarily of the same composition as that describedin step 1. For example, the added material may be pure calcined bauxiteor alumina when starting ingredients include uncalcined or partiallycalcined bauxites and/or clays; thus the composition of the pellets mayvary with radius. The addition of more dry ceramic powder is followed byrotating the impeller at a tip speed of between about 5 and about 20meters per second, preferably, between about 10 and about 20 meters persecond, for from about 1 to about 6 minutes, while continuing to rotatethe table at from about 10 to about 60 rpm and, preferably, from about20 to about 40 rpm. This step improves yield and results in improvedsphericity of the pellets.

If desired, the rotation of the impeller may then be stopped while thetable continues to rotate for between about 1 and about 5 minutes.

The impacting impeller is preferably a disk provided with peripheralrods or bars attached to the disk. The longitudinal axis of the rods orbars is desirably essentially parallel with the axis of rotation of theimpeller, which is usually a vertical axis. The diameter of the impelleris measured from the axis of rotation to the center of the most distantrod or bar. Tip speed is the speed of the most distant rod or bar.

The diameter of the impeller depends upon the size of the mixer but isusually slightly less than 25 percent of the diameter of the mixer. Theimpeller in most applications is between 10 and 100 centimeters indiameter and usually rotates at from 200 to 3,750 rpm at the lower tipspeeds of 10 to 20 meters per second, depending upon impeller diameter,and at from 500 to 6,500 rpm at the higher tip speeds of 25 to 35 metersper second, depending upon impeller diameter.

The pellets are screened for size, preferably after drying. However,they may be screened before drying or after furnacing. The rejectedoversized and undersized pellets and powdered material obtained afterthe drying and screening steps may be recycled. The finished pellets maybe tumbled to enhance smoothness. The resultant sintered pellets have abulk density ranging from about 1.35 to about 1.80 grams per cubiccentimeter, depending upon the ceramic starting ingredients employed.

The overall particle size of the pellets recommended for use as proppingagent for increasing permeability in a subterranean formation penetratedby well is between 0.1 and about 2.5 millimeters and preferably betweenabout 0.15 and 1.7 millimeters.

EXAMPLE 1

The pellets which may be produced according to this example correspondto those identified as Sample No. 1 in Table III. About 81 kilograms ofdiaspore clay material that had been previously calcined at atemperature sufficiently high to remove any organic materials andsubstantially all of the water of hydration from the clay, together withabout 54 kilograms of bauxite (60 percent by weight calcined clay 40percent by weight calcined bauxite) powder having an average particlesize of between 4 and 8 microns is added to an Eirich mixer having atable diameter of about 115 centimeters, an operating capacity of about160 kilograms and an impacting impeller diameter of about 27centimeters.

The table is rotated at about 35 rpm, and the impeller is rotated atabout 1,090 rpm, and about 27 kilograms of water is added. Rotation ofthe table and impeller is continued for about 1 minute; subsequently,the impeller speed is increased to about 2,175 rpm. The table andimpeller is rotated until seed pellets are formed, less than 5 percentof which being of a size smaller than 0.50 mm (about 3 minutes). Theimpeller is then reduced to about 1,090 rpm, and about 4.08 kilograms ofthe initial diaspore clay--bauxite powder mixture is added. Rotation ofthe pan and impeller is then continued for an additional 2 minutes toform spherical pellets.

The pellets are then dried for about 20 minutes at about 110° C. (230°F.) in a rotary dryer and then fired at about 1,480° C. (2,696° F.) forabout 5 minutes to sinter them. The yield of useful pellets having asize between 150 and 1,700 microns (0.15 and 1.7 millimeters) istypically greater that 80 percent by weight of starting ceramic powder.The resulting pellets have an apparent specific gravity of about 3.27, abulk density of 1.79 gm/cm³ and a sphericity of greater than 0.7, asdetermined using the Krumbein and Sloss chart. These pellets areidentified as Sample No. 1 in the accompanying Table III.

The pellets identified as Sample Nos. 2-29 in Table III were prepared ina manner comparable to that given in Example I except for the use ofdifferent ingredients and proportions and sintering temperatures.

In accompanying Table III is summarized the results of testing forseveral different pellets according to the invention which were producedfrom the raw materials in the proportions indicated. Unless otherwiseindicated, parts and percentages are by weight. All samples wereprepared in accord with the procedures given herein. Example 1 gives indetail the procedure employed in the preparation of Sample No. 1, whichprocedure is typical of that employed in preparation of the remainder ofthe samples reported in Table III.

The test procedures and apparatus employed to test the permeability indarcies of the pellets of the present invention including placing apredetermined quantity of the material to be tested between two parallelhardened steel plates, applying force tending to close the initiallyestablished 0.125 inch gap between the plates, then measuring the flowrate through the packed cell using distilled water at room temperatureat various applied stresses or pressures. The particle size distributionof the pellets was 20×40 mesh, U.S. Standard Sieve (ninety percent byweight minimum of pellets will pass through 20 mesh [841 micron] screenbut not through 40 mesh [420 micron] screen).

Values obtained using the American Petroleum Institute (API) procedurefor determining resistance to crushing are also reported in Table III.According to this procedure, a bed of about 6 mm depth of sample to betested is placed in a hollow cylindrical cell. A piston is thereafterinserted in the cell. Thereafter, a load is applied to the sample viathe piston. One minute is taken to reach maximum load which is then heldfor two minutes. The load is thereafter removed, the sample removed fromthe cell, and screened to separate crushed material. The results arereported as the weight percentage the original sample that crushed. Thiscrushed material is referred to in Table III as "percent finesgenerated. "

Single Pellet Crush is another procedure useful in determining theresistance of pellets to an applied load. In this procedure the pelletsof a given sample are first separated into size fractions by sieving.United States Standard Sieves were employed for the data reported inTable III. Thereafter, for each size fraction a quantity, eg. abouttwenty (20), of the individual pellets are measured and recorded. Eachindividual pellet is thereafter placed between two flat, hardenedparallel plates which are mounted in a press. A load is thereafterapplied to the plates. The amount of load required to crush a specificpellet is noted. Results are reported as the arithmetic average valueobtained within each size fraction of a given pellet sample for theamount of applied load (pounds force) divided by the diameter-in-inchessquared for each of the individual pellets tested (load/diametersquared). The use of ceramic plates is recommended to minimize wear ofthe testing apparatus.

Carborundum Crush is another procedure for determining the resistance ofpellets to an applied load. In this procedure a steel die having acylindrical cavity of one and one-eighth inch diameter (one square incharea) is filled 10 grams of the pellets to be tested. Thereafter a rightcylindrical plunger closely corresponding in diameter to that of the dieis inserted into the die and loaded by manually pumping a hydraulicpress. The results are reported as the amount of compression (originalunloaded pellet column height minus loaded pellet column height) versusapplied load. The amount of compression is expressed in thousandths ofan inch. The load is expressed in psi.

Acid solubility of the samples reported in Table III was tested inaccordance with recommended API test procedure, fourth draft, June 1980.In this procedure a known weight of sample (5 g) is placed in a 150 mlpolyethylene beaker containing 100 ml of a combined acid solutioncontaining 12 percent HCl acid and 3 percent HF acid. Thesample-and-acid-containing beaker is then placed in a 65.6° C. (150° F.)water bath for 30 to 35 minutes. The sample is not stirred. The sampleis thereafter filtered through a previously weighed filter crucible orfunnel and washed three times with 20 ml portions of distilled water.The filtered and washed sample is thereafter dried to constant weight(approximately one hour) at 105° C. (220° F.). The values given in TableIII represent the percentage of weight lost or dissolved due to theacid.

Sphericity of the pellets reported in Table III was determined using aKrumbein and Sloss chart. The values reported represent an average of10-20 pellets per sample.

Roundness as reported in Table III is a measure of the relativesharpness of the pellet corners, or of curvature. This evaluation may bedone at the same time and on the same sample as that used forsphericity. The pellets are visually compared with a Krumbein and Slosschart. The values reported represent an average of 10-20 pellets persample.

Bulk density values reported in Table III were determined by weighingthat amount of sample that would fill a cup of known volume.

    TABLE III      SAMPLE NO. 1 2 3 4 5 6 7 COMPOSITION   60% Calcined  4% D.C. Fines D.C.     Fines 60% Calcined Clay 50% Calcined Clay   Diaspore Clay Sintered 76%     Calcined Clay 72% Calcined Clay 10% Dust Collector 10% Dust Collector     70% Dust Collector  40% Calcined Calcined 20% Calcined 20% Calcined     Fines 30% Calcined Fines 40% Calcined Fines 30% Calcined  Bauxite     Bauxite Bauxite Bauxite Bauxite Bauxite Bauxite       SINTERING TEMP. (C.°)  1480  1495   1435 1410  1480       1400-1500 ROUNDNESS  0.86 0.82 -- 0.78 0.78 0.81 Repeat Samples     SPHERICITY  0.85 0.87 -- 0.79 0.80 0.81 Failed Initial ACID SOLUBILITY     (12-3)  5.42 1.60 7.09 6.92 6.33 5.11 Strength Quali- BULK DENSITY     (g/cc) 1.79 2.05 1.62 1.64 1.71 1.67 fication Tests  (pcf) 112 128  101     102 107 104.00  Very Poor Product ASG (g/cc)  3.27 3.58 3.05 3.02 3.05     3.02 API CRUSH (7500 psi) -- -- -- -- -- -- (% weight (10000 psi)  9.9     3.8  8.4  7.50 7.42 7.19 of fines generated) SINGLE PELLET (-16/+20) --     26542  25974 22852  -- 23427 CRUSH (-20/+25) 30903  28990  22481 25415     -- 24158  (-25/+30) 31021  -- 27985 28757  -- 27365  (-30/+35) 34011     34250  30319 30604  -- 28703  (-35/+40) 30827  33379  -- 34717  -- 30599     PERMEABILITY 2000 225 229 -- 214 235 (Darcies) 4000 192 208 -- 194 210     at 6000 162 188 -- 166 180 Applied 8000 136 165 -- 135 140 Pressure     10000  110 144 --  99 104 (psi) 12000   86 123 --  76  82  14000   64     102 --  62  66 CARBO CRUSH 0.010 inch (force 0.020 inch applied in 0.030     inch lbs × 1000 0.040 inch to induce 0.050 inch specified 0.060     inch deflection) 0.070 inch       SAMPLE NO. 8 9 10 11 12 COMPOSITION   5% Dust Collector 5% Uncalcined     Clay 5% Uncalcined Clay    Fines Remainder Addition (1982 Stockpile)     Addition (1982 Stockpile) Control Batch Control Batch Control Batch     Remainder Control Remainder control (Refer to Table I) (Refer to Table     I) Material Batch Material Batch Material       SINTERING TEMP. (C.°)  1465 1480 1495 1510 1465 1480 1495 1510     1480 1495 1510 1465 1485 1495 1510  1465 1485 1495 1510 ROUNDNESS     SPHERICITY ACID SOLUBILITY (12-3)      3.89 BULK DENSITY (g/cc)  (pcf)     ASG (g/cc)   3.20  3.22  3.22  3.22  3.20  3.21  3.21  3.21 3.10 3.09     3.10 3.14 3.12 3.12 3.13 3.14 3.13  3.13  3.13 API CRUSH (7500 psi) (%     weight (10000 psi)   7.00  6.02  4.69  5.26  7.90  6.06  3.96  4.25 7.45     6.77 5.98 10.32 9.86 8.34 7.34 8.43 7.01  6.75  6.50 of fines generated)     SINGLE PELLET (-16/+20) CRUSH (-20/+25)  (-25/+30)  (-30/+35)  (-35/+40)     PERMEABILITY 2000 (Darcies) 4000 at 6000 Applied 8000 Pressure 10000     (psi) 12000   14000  CARBO CRUSH 0.010 inch  1.4  1.8  2.0  2.0  1.4     2.0  2.2  2.0 2.0 2.0 2.4 1.6 2.0 2.0 2.2 2.0 2.4  2.6  2.6  0.020 inch     2.8  4.0  4.8  4.0  2.8  4.2  5.0  5.0 4.6 5.0 6.2 4.2 4.6 5.6 6.2 5.5     6.4  5.6  5.8  0.030 inch  6.6  9.0  9.8  8.4  6.6  9.6 10.2  9.6 10.2     10.8 11.2 8.0 8.6 9.4 10.2 8.0 10.4 10.4 10.4  0.040 inch 10.4 14.2 14.6     14.6 10.4 14.2 15.2 14.8 14.0 14.8 15.6 11.6 12.4 13.0 13.8 11.6 13.8     14.0 14.2  0.050 inch 14.2 18.4 18.6 18.0 14.2 18.6 19.0 18.4 17.8 18.4     20.0 19.6 15.4 16.8 17.2 14.6 17.6 18.6 18.6  0.060 inch 18.0 23.2 23.6     21.6 18.0 23.6  24+ 22.8 20.8 22.4 24.0 17.4 18.2 19.2 20.2 18.0 20.8     22.8 22.6  0.070 inch 21.0 24.0 24.0 23.4 21.0  24+ --  24+ 22.8 24.0+     -- 20.4 21.6 22.0 23.0 21.8 23.2  24+      24+                                 SAMPLE NO. 13 14 15 16 17 18     COMPOSITION  5% Uncalcined      Clay Addition (16 5% Uncalcined Clay 10%     Uncalcined Clay  20% Uncalcined Clay 5% Uncalcined Clay Mesh and Finer)     Addition (16 Mesh Addition (1982 15% Uncalcined Clay Addition (1982     Addition (1982 Stockpile) Remainder Con- and Finer) Remain- Stockpile)     Addition (1982 Stockpile) Stockpile) Remainder Control trol Batch der     Control Batch Remainder control Remainder Control Remainder Control     Batch Material Material Material Batch Material Batch Material Batch     Material       SINTERING TEMP. (C.°)  1465 1485 1495 1510 1465 1480 1495 1465     1480 1495 1495 1465 1480 1495 1510  1465 1480 1495 1510 ROUNDNESS     SPHERICITY ACID SOLUBILITY (12-3) BULK DENSITY (g/cc)  (pcf) ASG (g/cc)  3     .14 3.13  3.12  3.13 3.15 3.13 3.13 3.16 3.14 3.14 3.11 3.09  3.08  3.09       3.09 3.08 3.04 3.04 3.04 API CRUSH (7500 psi) (% weight (10000 psi)     10.7 7.16  6.85  6.34 10.1 8.35 8.49 11.8 9.86 9.95 7.88 9.55  8.26     7.59  6.72 10.12 8.76 7.83 7.56 of fines generated) SINGLE PELLET     (-16/+20) CRUSH (-20/+25)  (-25/+30)  (-30/+35)  (-35/+40) PERMEABILITY     2000 (Darcies) 4000 at 6000 Applied 8000 Pressure 10000  (psi) 12000     14000  CARBO CRUSH 0.010 inch 2.0 2.0  2.4  2.2 2.0 2.2 2.4 2.0 2.0 2.0     1.6 1.8  2.0  2.0  2.0 1.8 2.2 (force 0.020 inch 5.0 5.2  6.0  6.2 5.0     5.4 5.0 5.0 4.4 4.6 4.0 3.8  5.0  5.0  5.2 3.4 5.0 applied in 0.030 inch     8.6 9.2 10.0 10.4 8.0 9.0 9.0 6.8 7.4 8.0 7.4 8.0 10.0 10.2 10.2 8.0     10.2 lbs × 1000 0.040 inch 11.6 12.7 14.0 14.6 10.6 12.0 16.8 9.6     10.2 10.6 10.4 11.0 13.2 13.6 14.6 11.4 13.2 to induce 0.050 inch 15.0     15.8 17.6 18.4 13.0 15.0 16.0 11.6 12.6 12.8 14.8 14.4 17.0 17.4 19.0     13.8 17.0 specified 0.060 inch 17.6 18.4 21.8 23.2 15.0 18.0 19.2 14.0     15.0 15.4 17.8 17.0 20.0 21.0 22.8 17.4 19.6 deflection) 0.070 inch 21.2     22.8  24+  24+ 19.0 22.0 22.8 17.0 19.2 19.8 21.8 20.6  24+  24+  24+     20.6 23.2       SAMPLE NO. 19 20 21 22 23 24 COMPOSITION 30% Uncalcined Clay 40%     Uncalcined Clay 52% Uncalcined 50% Uncalcined 50% Uncalcined 60%     Uncalcined Addition (1982 Stockpile) (1982 Stockpile) Clay (Stockpile     Clay (Stockpile Clay (Stockpile #1) Clay (Stockpile Remainder Control     60% Calcined #1) 50% Calcined #1) 50% Calcined 50% Calcined #1) 40%     Calcined Batch Material Bauxite Bauxite Bauxite Bauxite Bauxite       SINTERING TEMP. (C.°)  1465 1480 1495 1510 1450 1465 1480 1495     1450 1465 1480 1450 1465 1480 1440  1450 1465 1450 1465 ROUNDNESS     SPHERICITY ACID SOLUBILITY (12-3)          4.35 BULK DENSITY (g/cc)     (pcf) ASG (g/cc)  3.03 2.98 2.99  2.97 3.11  3.09 3.09 3.09 3.17 3.17     3.15 3.13 3.11 3.09 3.14 3.13 3.11 3.00 3.00 API CRUSH (7500 psi) (%     weight (10000 psi)  11.98 8.61 8.50  9.04 6.72  7.00 8.26 8.78 9.65 8.20     7.45 10.52 7.60  8.98 7.41 6.90 12.14 14.21 of fines generated) SINGLE     PELLET (-16/+20)  (-20/+25)  (-25/+30)  (-30/+35)  (-35/+40) PERMEABILITY      2000                 260 (Darcies) 4000                 224 at 6000                 189 Applied 8000                 153 Pressure 10000             117.3 (psi) 12000                  81.5  14000     45.7 CARBO CRUSH 0.010 inch 3.2 4.0 4.0  5.0   2.6 2.2     3.3 3.1     2.7 2.3 (force 0.020 inch 7.0 8.2 8.0 10.0   5.6 4.8     6.1 5.8     5.3     4.7 applied in 0.030 inch 10.0 11.1 11.0 14.0  11.2 10.0     8.8 8.4     7.4 6.5 lbs × 1000 0.040 inch 12.6 13.8 14.0 17.2  14.6 13.0     11.7 10.4     9.6 8.9 to induce 0.050 inch 14.6 16.2 16.4 20.6  17.8     17.4     14.9 13.9     12.2 11.3 specified 0.060 inch 16.6 20.6 20.0     23.0  21.2 19.4     18.6 16.9     14.8 7.4 deflection) 0.070 inch 20.6     23.2 23.8  24+      24+ 22.8     21.7 19.8     17.9 17.0                      SAMPLE NO. 25 2     6 COMPOSITION  Uncalcined 60% Uncalcined Clay Clay (Stockpile (Stockpile     #1) #1) 30% Calcined  40% Calcined Bauxite Bauxite       SINTERING TEMP. (C.°)  1450 1400 1370  1350 1440 1450 1460     ROUNDNESS SHPERICITY ACID SOLUBILITY (12-3) BULK DENSITY (g/cc)  1.52     1.59 1.66 1.75  (pcf) ASG (g/cc)   2.82 2.88 3.06 3.08  2.92  2.93  2.80     API CURSH (7500 psi) (% weight (10000 psi)  15.32 12.4  8.70 9.29 11.78     11.58 14.71 of fines generated) SINGLE PELLET (-16/+20)  (-20/+25)     (-25/+30)  (-30/+35)  (-35/+40) PERMEABILITY 2000   249 (Darcies) 4000     219 at 6000   189 Applied 8000   159 Pressure 10000    130 (psi) 12000      100  14000     70 CARBO CRUSH 0.010 inch (force 0.020 inch applied in     0.030 inch lbs × 1000 0.040 inch to induce 0.050 inch specified     0.060 inch deflection) 0.070 inch

Referring to Table III, Sample Nos. 1 and 2 may be used as baselines forcomparison with pellets prepared according to the invention. Sample No.1 was prepared from 60 percent calcined diaspore clay and 40 percentcalcined bauxite and is considered to be an intermediate strengthproduct suitable for use in wells down to a depth of about 14,000 feet,which corresponds to pressures of about 700 kg/cm² (10,000 psi). Sample2 was prepared from calcined bauxite and is considered to be a premiumhigh strength product suitable for use in wells of a depth exceeding14,000 feet. Sample No. 2 is the standard against which other productsincluding other standards such as Sample No. 1 are compared.

Samples Nos. 8 and 9 were prepared from a control batch of calcined clayand bauxite ingredients having the analysis indicated in Table 1. Thecontrol batch material is standard employed to assure that processingconditions are being maintained. The chemical composition of the controlbatch is similar to that of Sample No. 1, Table III.

Samples Nos. 3-7 and 10-26 of Table III were prepared utilizing at leastone uncalcined or partially calcined ingredient.

The data given in Table III with respect to Sample Nos. 3, 4, 5, 6, 10when compared with Sample No. 1 supports a conclusion that Sample Nos.3, 4, 5, 6, 10 made from ingredients, including dust collector fines,are suitable as intermediate strength proppants. It is notable that theapparent specific gravity of Sample Nos. 3-6 and 10 are lower than thatof Control Sample Nos. 1 and 8-9, yet the resistance-to-crushing asdirectly indicated by any of the crush resistance test results orindirectly indicated by the permeability test results of Sample Nos. 3-6and 10 are comparable to that of the controls.

Sample No. 7 was prepared using 70 percent dust collector fines. Thisproduct was unsatisfactory and crushed easily.

Samples Nos. 11-15 were each prepared from the same ratio and type ofingredients (5 percent uncalcined clay, remainder control batch). Thesesamples demonstrate the variations in product characteristics that canbe expected. These samples are within the invention and are deemedsuitable for use as intermediate strength proppants. These samples alsoexhibit lower apparent specific gravities than control samples 1 and8-9.

Sample Nos. 11-25 were prepared from using different amounts and sourcesof uncalcined clay. Each of these samples is deemed suitable for use asproppants.

Generally speaking, as the amount of uncalcined clay is increased, theapparent specific gravity of the resulting pellets is reduced andeffective sintering may be done at a lower temperature. Sample No. 26 isdeemed marginally suitable due to its high level of fines generated onAPI Crush.

A comparison of the test results for Samples Nos. 1 and 5 indicates thatsubstitution of 10 percent dust collector fines for calcined bauxitewhen the remaining ingredient is clay actually improves characteristicsof the pellets that are considered significant for proppants. Note thatSample 5 exhibited lower apparent specific gravity, lower percent finesgenerated on API Crush and greater permeability than Sample No. 1. Thepermeability of Sample Nos. 1 and 5 was comparable. This is surprising.It was previously thought that the use of a higher amount of silica andmaterial having a high LOI (due to substitution of dust collector finesfor calcined bauxite) would result in a lower performance product due tothe lower amount of alumina and greater amount of closed porosity asevidenced by lower apparent specific gravity. It was thought that use ofa material having a high LOI (eg. greater than 1-2 percent) would damagethe structure as the chemically bound water and volatiles are driven offduring furnacing of the pellets.

Referring now to Sample Nos. 8 and 9 of Table III, it can be seen thatsintering temperature has a significant effect on the strength ofpellets of a given composition. For Sample Nos. 8 and 9 furnacing thedried pellets at 1495° C. provided greatest resistance to crushing asevidenced by lowest percent lines generated for API Crush and greatestamount of applied load to induce a specified compression. A value of 24+or lack of an entry following a 24+ entry indicates that the capacity ofthe press was reached or would have been exceeded for the specifiedamount of compression.

A comparison of the results given in Table III for Sample Nos. 1 and 25reveals that use of 60 percent uncalcined clay permits furnacing atlower temperature while yielding a product of lower apparent specificgravity, lower API Crush and greater permeability.

The composite, spherical, sintered pellets of the present invention areuseful as a propping agent in methods of fracturing subterraneanformations to increase the permeability thereof, particularly thoseformations having a compaction pressure of at least 280 kg/cm² (4000psi), which are typically located at a depth 6,000 feet or greater.Pellets according to the present invention are presently believed to beparticularly suitable for use at depths greater than 7,000 feet but lessthan 14,000 feet.

When used as a propping agent, the pellets of the present invention maybe handled in the same manner as other propping agents. The pellets maybe delivered to the well site in bags or in bulk form along with theother materials used in fracturing treatment. Conventional equipment andtechniques may be used to place the spherical pellets as propping agent.

A viscous fluid, frequently referred to as "pad", is injected into thewell at a rate and pressure to initiate and propagate a fracture in thesubterranean formation. The fracturing fluid may be an oil base, waterbase, acid, emulsion, foam, or any other fluid. Injection of thefracturing fluid is continued until a fracture of sufficient geometry isobtained to permit placement of the propping pellets. Thereafter,pellets as hereinbefore described are placed in the fracture byinjecting into the fracture a fluid into which the pellets havepreviously been introduced and suspended. The propping distribution isusually, but not necessarily, a multi-layer pack. The overall particlesize of the pellets is between about 0.1 and about 2.5 millimeters and,more preferably, between about 0.15 and about 1.7 millimeters. Followingplacement of the pellets, the well is shut-in for a time sufficient topermit the pressure in the fracture to bleed off into the formation.This causes the fracture to close and apply pressure on the proppingpellets which resist further closure of the fracture.

The foregoing description and embodiments are intended to illustrate theinvention without limiting it thereby. It will be understood thatvarious modifications can be made in the invention without departingfrom the spirit or scope thereof.

We claim:
 1. A composite, sintered, spherical pellet suitable for gasand oil well proppant which is prepared from materials consistingessentially of clay materials, bauxitic materials and alumina, whereinsaid materials include at least one uncalcined or partially calcinedingredient selected from the group consisting of clay materials,bauxitic materials and dust collector fines produced during thecalcination of clay materials and bauxitic materials, and alumina, saidpellet having on a dry weight basis an alumina-to-silica ratio fromabout 9:1 to about 1:1 and at least about 6.51 percent by weight ofother than alumina and silica, said pellet having an apparent specificgravity of less than 3.30.
 2. The pellet of claim 1 wherein a pluralityof said pellets has a permeability to distilled water at about 75° F.(24° C.) which decreases not more than about three-fourths when thepressure applied to said pellets is increased from 2000 to 10,000 psi(140-700 kg/cm²).
 3. The pellet of claim 1, wherein the pellet has analumina-to-silica dry weight basis ration from about 4:1 to about 6.5:1.4. The pellet of claim 1, wherein the pellet has an alumina-to-silicaratio of about 5:1.
 5. The pellet of claim 1, 2 or 3 wherein said pelletis of an overall size ranging from about 0.1 to 2.5 millimeters and anapparent specific gravity from about 2.6 to 3.3.
 6. The pellet of claim1, wherein the pellet is at least 85 percent by weight of Al₂ O₃ andSiO₂.
 7. The pellet of claim 1, wherein the pellet is not more thanabout 82 percent by weight of Al₂ O₃.
 8. The pellet of claim 1 whereinthe pellet is prepared from a mixture containing on a dry weight basisfrom about 20 to about 40 percent calcined bauxite, from about 4 toabout 10 percent dust collector fines with the remainder being calcinedclay.
 9. The pellet of claim 1 wherein the pellet is prepared from amixture containing on a dry weight basis from about 5 to about 30percent uncalcined clay with the remainder being calcined bauxite andcalcined clay.
 10. The pellet of claim 1 wherein the pellet is preparedfrom a mixture containing on a dry weight basis from about 40 to about70 percent uncalcined clay with the remainder being calcined bauxite.11. A sintered, spherical composite pellet suitable for use as gas andoil well proppant, said pellet having an Al₂ O₃ /SiO₂ dry weight basisratio from about 9:1 to about 1:1, said pellet having an apparentspecific gravity of less than 3.30 g/cc and at least about 6.51 percentby weight of other than Al₂ O₃ and SiO₂, said pellet being made by aprocess including the steps of:(a) Forming a particulate mixture frommaterials consisting essentially of clay materials, bauxitic materialsand alumina wherein said materials include at least one uncalcined orpartially calcined ingredient selected from the group consisting of claymaterials, bauxitic materials and dust collector fines produced duringthe calcination of clay materials and bauxitic materials and alumina,having an average particle size of less than 15 microns in a highintensity mixture and at least about 6.51 percent on a dry weight basisother than Al₂ O₃ and SiO₂ ; (b) while stirring the mixture, addingsufficient water to cause formation of composite spherical pellets fromthe mixture; (c) drying the pellets until less than three (3) weightpercent free moisture remains in the pellets; (d) furnacing the driedpellets at a furnace temperature ranging from about 1300° C. to about1500° C. for a period sufficient to enable recovery of sintered,spherical, composite pellets having a bulk density from about 1.35 to1.8 grams per cubic centimeter.
 12. The pellet of claim 11, wherein themixture is intensively stirred during step (b).
 13. The pellet of claim11, wherein the particulate mixture has an average particle size of lessthan about 10 microns.
 14. The pellet of claim 11, further comprising,prior to step (c), adding from about 5 to about 15 percent by weight ofpellets of any of the ingredients of the mixture of step (a).
 15. Thepellet of claim 11 wherein the dried pellets are furnaced for a periodfrom about 4 to about 8 minutes.
 16. The pellet of claim 15 whereinprior to step (c) there is added from about 5 to 15 percent by weight ofpellets of the mixture of step (a) while continuing to stir the pellets.17. A plurality of pellets as described in claim 1, having a bulkdensity of between about 1.35 and 1.80 grams per cubic centimeterwherein said pellets are of a diameter from about 0.1 to about 2.5millimeters.
 18. A process for manufacturing composite, sintered,spherical pellets suitable for use as gas and oil well proppant havingan alumina-to-silica ratio on a dry-weight-basis from about 9:1 to about1:1 and at least about 6.51 percent by weight of other than alumina andsilica, said process comprising the steps of:(a) adding to ahigh-intensity mixer, in predetermined ratio, starting ingredientsconsisting essentially of clay materials, bauxitic materials and aluminawherein said materials include at least one uncalcined or partiallycalcined ingredient selected from the group consisting of claymaterials, bauxite materials and dust collector fines produced duringthe calcination of clay materials and bauxitic materials, to materialsconsisting essentially of calcined clay materials and/or a member of thegroup of calcined bauxite materials, alumina or mixtures thereof, eachingredient having an average particle size of less than 15 microns; (b)stirring said starting ingredients to form a particulate mixture havingan average particle size of less than 15 microns and at least about 6.51percent on a dry weight basis of other than alumina and silica; (c)while stirring the mixture, adding sufficient water to cause formationof composite, spherical pellets from the mixture; (d) drying the pelletsuntil less than ten (10) weight percent free moisture remains in thepellets; (e) furnacing the dried pellets at a furnace temperature offrom about 1300° C. to about 1500° C. for a period sufficient to enablerecovery of sintered, spherical, composite pellets having a bulk densityof from about 1.35 to about 1.80 grams per cubic centimeter and at leastabout 6.51 percent by weight of other than alumina and silica.
 19. Theprocess of claim 18 wherein the pellets are dried at a temperaturebetween about 100° C. and about 300° C. until less than three (3) weightpercent free moisture remains in the pellets.
 20. The process of claim18 wherein each ingredient of the particulate mixture has an averageparticle size of less than 10 microns.
 21. The process of claim 18,further comprising, prior to step (d), adding from about 5 to about 15percent of any of the ingredients given in step (a).
 22. The process ofclaim 18, wherein the dried pellets are furnaced for a period from about4 to about 20 minutes.
 23. The process of claim 18, further comprising,prior to step (d), adding from about 5 to 15 percent of any of theingredients given in step (a) while stirring the pellets.
 24. A gas andoil well proppant comprising a plurality of composite, sintered,spherical pellets having a permeability to distilled water at about 75°F. (24° C.) which decreases not more than about three-fourths when thepressure applied to said pellets is increased from 2000 to 10,000 psi(140-700 kg/cm²), said pellets being prepared from, on a dry weightbasis, materials consisting essentially of clay materials, bauxiticmaterials and alumina wherein said materials include at least oneuncalcined or partially calcined ingredient selected from the groupconsisting of clay materials, bauxitic materials and dust collectorfines produced during calcination of clay materials and bauxiticmaterials, and alumina, or mixtures thereof, said pellets having analumina-to-silica dry weight basis ratio from about 9:1 to about 1:1,said pellets having an apparent specific gravity of less than 3.30 andat least about 6.51 weight percent of other than alumina and silica anda diameter between about 0.1 and about 2.5 millimeters.
 25. The pelletof claim 1 wherein said pellet is prepared from a mixture consistingessentially of clay materials and bauxitic materials.
 26. The method ofclaim 18 wherein fluxes are not added to the high-intensity mixer.